In the field of electronics assembly manufacturing, it is often necessary to monitor work pieces at a particular manufacturing stage, in order to determine qualitatively or quantitatively how a process used at that stage is performing. One example of such a process is the process of soldering components to a PC board. During a conventional PC board soldering process, a solder stencil is placed over a component-connection surface of a PC board. The solder stencil has numerous openings corresponding to locations where solder paste is to be placed, such as contact pads that will subsequently receive component leads. After the solder stencil has been placed, solder paste is deposited over it. Then the solder stencil is removed, leaving the PC board stenciled with numerous blocks of solder paste on its surface. Once the board has been stenciled in this fashion, the components are placed on the board, and the assembly is heated to reflow the solder, thereby completing the connection between the components and the board.
In the foregoing soldering process, it is necessary to monitor how well the solder "wets" or covers its associated pads and component leads. Poor wetting in particular can be problematic, as it may contribute to defective solder joints between the component and the board. Poor wetting may be caused by a variety of factors, such as improper oven temperature or poor solder paste quality, for example. Another potential problem is the tendency of the solder stencil to become clogged with solder as it is used repetitively. As a stencil becomes clogged, less solder is deposited on the board; this may also contribute to defective solder joints. For these reasons, then, monitoring of the soldering process is essential to achieving good PC board yields.
Another example of monitoring an electronics assembly manufacturing process is the monitoring of electronic component placement. Since much of present-day placement is performed automatically, it is necessary to monitor the performance of placement equipment so that adjustments can be made if necessary. In particular, monitoring may involve determining whether all the necessary components have been attached to a given PC board, and whether they are correctly aligned with interconnection pads on the board so that good electrical contact is assured.
A common method of carrying out necessary process monitoring is to visually inspect the boards upon completion of the process step being monitored. To monitor the degree of solder wetting, for example, a technician might examine a board in several places, looking for evidence of insufficient wetting. This process is tedious, error-prone, and time-consuming, and does not necessarily yield consistent results. Improved methods of inspection are therefore desirable that can be performed as quickly and robustly as possible, so that effective monitoring is obtained at a minimum cost.